Magnetic steel process

ABSTRACT

EACH FACE OF A SHEET OR STRIP OF MAGNETIC STEEL IS COATED WITH A THIN LAYER OF AT LEAST ONE ELEMENT OF ALUMINUM, SILICON, COBALT AND GERMANIUM, WHICH ELEMENT IS DIFFUSED INTO THE SHEET UNTIL THE ELEMENT IS DISTRIBUTED THEREIN IN A CONCENTRATION DECREASING FROM THE FACES TO THE CORE OF THE SHEET, AND THE RESULTING PRODUCT IS SUBMITTED TO HEAT TREATMENT UNDER THE INFLUENCE OF A MAGNETIC FIELD.

United States Patent Int. 01. time 7/02 U.S. Cl. 117-234 10 ClaimsABSTRACT OF THE DISCLOSURE Each face of a sheet or strip of magneticsteel is coated with a thin layer of at least one element of aluminum,silicon, cobalt and germanium, which element is diffused into the sheetuntil the element is distributed therein in a concentration decreasingfrom the faces to the core of the sheet, and the resulting product issubmitted to heat treatment under the influence of a magnetic field.

This application is a continuation-in-part of Ser. No. 655,005, filedJuly 21, 1967, and now abandoned.

The present invention relates to a method of improving the magneticproperties of magnetic steels for magnetic applications, and also to theproducts obtained thereby.

Manufacturers of electrical equipment are becoming more and moreparticular about the efficiency of electrical machines such astransformers, alternators, etc., and this has led steel specialists toendeavour to develop products in the form of thin sheets for use inconstruction of magnetic circuits of these machines having constantlyimproved properties, notably increasingly lower watt losses, since it isthe loss of watts in the magnetic circuit that governs the no-loadefficiency of a machine and constitutes a constant drawback during theentire useful life of the machine due to the electric power dissipatedand lost in the form of heat.

As a rule, the total losses of this character are subdivided intohysteresis losses and eddy-current losses, although such subdivision isdisputable from a purely theoretical point of view.

Upon considering the fact that all ferro-magnetic materials consist ofjuxtaposed elementary Weiss domains, no difference in character shouldexist between hysteresis losses and eddy-current losses, since bothresult from induced current at a suitable scale.

To reduce losses, it is particularly advantageous to make these inducedcurrents as low as possible. To this end, two solutions may becontemplated, i.e. either increasing the resistivity or specificresistance of the metal, or in the alternative, modifying in a suitabledirection the configuration of the Weiss domains.

The first technique was used from the beginning of this century. Siliconand/or aluminum, or any other suitable elements are added to the ironwith a view toward increasing the resistivity of the resulting material.

Another much more recent technique consists of obtaining a metal texturehaving a predetermined orientation. In properly oriented industrialmaterials the configuration of the elementary Weiss domains tends to resemble that observed in monocrystals, without in any case attainingtheir simplicity and perfection.

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ma homogeneous substance having a given resistivity the perfection ofthe texture orientation constitutes an important factor in that itexerts a direct influence on the configuration of the elementary Weissdomains. It is in this particular manner that it controls all themagnetic properties of the substance, notably the magnitude of itslosses.

Japanese Pat. 38/ 18,068 relates to the manufacture of an aluminum steelby a process which includes deposition of aluminum on iron and adiffusion step. This patent, however, fails to mention the decreasingcontent of the coating element as required in the present invention. Thehighest concentration of the element near the surface of the steel sheetbrings about a substantial increase in resistivity, precisely in thearea where the currents induced in the sheet have the highest value.

Japanese Pat. No. 39/4,697 discloses a magnetic material coated with athin layer of metal having a greater magnetic permeability than thesubstrate. However, this process provides a composite product of twomaterials, i.e. one layer of material bonded to another. The articlesproduced by the process of this patent fail to provide for the diffusionof the metal into the magnetic material, so as to provide aconcentration gradient of the diffused metal.

U.S. Pat. 3,028,269 discloses the coating of certain metals, e.g. zincand aluminum, on loW carbon ferrous metal articles. However, there is nomention therein of the increase in magnetic properties of the coatedarticle, nor of the concentration gradient of the coating metal, i.e.the decrease in concentration of the zinc or aluminum from the surfaceto the core of the substrate. In fact, it is unlikely that suchconcentration gradient exists in the articles manufactured by theprocess of this patent since the temperature of the box anneal followingthe coating step is too low to cause diffusion of the coating metal.Thus, in the case of zinc, the box anneal is conducted at temperaturesup to 550 F. (about 288 C.), and in the case of aluminum, attemperatures up to 650 F. (about 344 C.). These relatively lowtemperatures (compared to 750-1050 C. in the present invention) wouldnot be sufiicient to cause diffusion of the zinc or aluminum into thesubstrate to cause a concentration gradient, and there fore there wouldbe no significant decrease in the watt losses of the ferrous metalarticle, as compared to the products produced in the present invention.

US. Pat. 2,410,220 is concerned with surface coating, wherein a metal isdeposited on a substate and the deposited metal is then oxidized toobtain an oxide which cannot be reduced by hydrogen. Thus, it isapparent that there is no diffusion of the deposited metal into thesubstrate, and therefore no concentration gradient.

Among the possible orientations two types provide more particularlyundeniable technical advantages. The first is the so-called Goss (110)[001] orientation or cube on edge or edged cubical structure, and thesecond is the so-called cubical [001] orientation or cube laid flat orflat cubical structure. In this case the Millerian indices are used fordesignating the orientation concerned.

Nowadays metallurgists are capable of manufacturing magnetic materialshaving these two structure patterns, and the so-called Goss texture madeit possible during the last few years to achieve remarkable improvementsin the construction of electrical machines in general.

It is an object of the present invention to provide a method wherebysubstantial improvements in the magnetic properties of steel sheets areobtained, notably in the form of a considerable reduction in the wattlosses in relation to those characterizing hitherto known steels formagnetic application.

In fact, this invention permits of obtaining magnetic steel sheetseither oriented according to a predetermined texture (such as edged orflat cubical), or non-oriented, containing an additive capable ofincreasing the resistivity, such as at least one of aluminum, silicon,cobalt and germanium, and any other element capable of exerting asimilar effect, at a concentration higher than the values attainableheretofore, with the result that the resistivity of the metal isincreased to a degree greatly exceeding that attainable up to thepresent.

The steel sheets subjected to the process of this invention are alreadyprocessed, and contain silicon, preferably in an amount of ODS-6% byweight. However, it is to be understood that the amount of silicon isnot critical, and is a function of the presence or absence of otherconstituent materials in the steel sheet. The English term for thesesilicon steel sheets is electrical steel sheets, whereas the French termis magnetic sheets, these terms bing well-recognized in the art, itbeing unnecessary to specify the proportion of silicon in the sheets.

As indicated above, the sheets subjected to the process of thisinvention are already in their processed state. The art is completelyunaware of any method whereby the magnetic properties of such processedsteel sheets having oriented silicon grains can be substantiallyimproved, by heating such sheets under specific conditions, followed bydeposition of the coating element on the sheet, and magnetic treatmentunder equal specific conditions. The present invention provides such asprocess, wherein the resultant products have vastly superior magneticproperties compared to those previously known.

The present invention is characterized mainly by the fact that itcomprises coating each face of a magnetic steel product occurringgenerally in sheet form, whether oriented or not, with a thin layer ofat least one element such as aluminum, silicon, cobalt and germanium,causing the element or elements to be diffused into the sheet whilepreserving the initial texture thereof until the added element orelements is or are properly distributed therein, at a concentration ratedecreasing from the surface to the core of the sheet, this distributionleading to the most marked improvement in the reduction of watt losses,and subsequently applying to the resultant steel sheet a heat treatmentunder the influence of a magnetic field in order to obtain animprovement in the configuration of the surface Weiss domains by aneffect of directional order. The total amount of element coated on thesteel sheet is preferably within the range 0.05-3.0% by weight of thesheet.

According to one embodiment of the method of this invention, whichprovides a pronounced improvement in. the magnetic properties of thesteel sheet, the ditfused element is distributed at a concentration ratedecreasing from the surfaces to the core of the sheet, irrespective ofthe type of element contemplated.

According to another embodiment of the method of this invention, anadequate distribution of the selected diffused element is efiected withdue regard for the nature of the element and the steel sheet thickness.

According to a specific arrangement of the method of this invention,each face of the sheet of the magnetic product is coated with a thinlayer of the addition element or elements by resorting to knownprocedure generally specific to the element being deposited, for examplefrom a volatile compound of the element to be deposited, or byevaporating the element in vacuo.

According to another specific arrangement of the method of thisinvention, the element coated on the magnetic product is difiused intothe sheet by preheating the coated sheet under time-temperatureconditions which are subordinate to the element to be deposited on thesheet, and also take into account the desired penetration depth of theelement and the components of the reducing atmosphere.

According to the form of actuation of this last arrangement of themethod of this invention the sheet is heated at a temperature on theorder of 750 C. to 1050" C. during a time period up to hours in anatmosphere of dry hydrogen.

During this heating step the element is diffused into the sheet andtherefore a beneficial enrichment of the initial magnetic properties ofthe product is obtained, the enrichment thus obtained causing on the onehand an increase in the specific resistance of the metal and on theother hand a substantial improvement in the efiiciency of a treatment ofdirectional order, with the result that watt losses are appreciablyreduced in the initial product while preserving the recrystallizationcharacteristics obtained during the normal processing or manufacture ofthe magnetic product.

It is known the Goss texture is obtained by effecting a secondaryrecrystallization at a temperature in excessof 1100 C. and that itcontains very few pattern defects, whereby a perfect stability isimparted thereto. Then aluminum or one of the other beneficial elementscan be diffused according to this invention, while preserving thisoriented texture. Only when the heating conditions are maintained withinthe defined limits is it possible to conserve the edged cubical or fiatcubical structure of the steel sheet.

This invention provides two additional and extremely advantageouseffects:

(1) For each total concentration of alloy elements there is an optimumdistribution of the element added by coating and diffusion. Thisdistribution is not a uniform concentration in the sheet thickness. Thisinvention requires a greater concentration of the element on the sheetfaces, this concentration decreasing towards the core of the sheet,since the major induced currents circulate on the faces where the Weissdomains of complex shapes are found, as they are induced by all thedefects or imperfections of the orientation and the nature of thecrystal surfaces, which domains have been given the name closingdomains. For instance, considering strips having a thickness of 0.35 mm.(.01375"), the diffusion treatment giving the optimum conditions is suchthat the product Dt expressed in C.G.S. units has a value of from 10- to10* sq. cm., wherein D denotes the coefiicient of diifusion in squarecentimeters per second of the element contemplated, at the treatmenttemperature, and t designates the diffusion treatment time in seconds.

(2) The surface enrichment of the magnetic product permits amodification in the surface or Wall energy that can be exploited forproducing oriented crystallization of a novel type.

In addition to the arrangements set forth hereinafter, this inventionfurther comprises other arrangements which will readily appear to thoseconversant with the art from the following more detailed description.

This invention is particularly concerned with a method for improving themagnetic properties of steels for magnetic applications and with theproducts derived therefrom according to the provisions set forthhereinabove, and also with the elements for carrying out the method andobtaining the products, and the assemblies in which the method isincluded and the products are used.

This invention will be better understood from the description givenhereinafter with reference to a typical specific embodiment of themethod constituting the subject matter of this invention, which is givenby way of illustration and should not be construed as limiting the scopeof the invention.

The initial material is a steel in sheet form prepared according tostandard manufacturing specification either with a Goss (110) [001]texture or with a cubical (100) [001] texture, this material having athickness ranging from .25 to .4 mm. .01 to ,4, and being submitted tothe following successive treatments:

The first operation consists in scouring the metal to remove themagnesium silicate layer formed during the final recrystallizationannealing. This scouring operation may be carried out in anitro-hydrofiuoric acid bath containing 15% of nitric acid and 4% ofhydrofluoric acid, at a temperature of 60 C. After the scouringoperation, the metal is washed and dried.

The second operation consists in coating the thus cleaned metal surfacewith the selected compound, in this instance aluminum. To this end, themetal is treated in a vacuumized enclosure according to the knownprocedure usually referred to as evaporation in vacuo, and an equalquantity of aluminum is deposited on each face of the steel sheet to betreated. The total quantity to be deposited on the two faces ispreferably at the most 3% by weight of the sheet to be treated, and mayadvantageously approximate 0.5% by weight. In this instance, the totalamount is 1.0% by Weight.

After the aluminum coating has been applied, the diffusion treatmentdefined hereinabove is carried out. This treatment consists in effectinga static annealing in a dry hydrogen atmosphere at a temperature of 900C. during a time period on the order of 3 hours.

Finally, the sheet material is treated in a magnetic field at atemperature of 600 C. to 300 C., preferably in a passage furnace'containing a dry hydrogen atmosphere, the intensity of the field beingapproximately 50 oersteds and its direction being parallel to thepreferential direction of easy magnetization of the sheet to be treated.Thus, for example, in the case of sheets having a Goss texture thedirection of the magnetic field must be parallel or substantiallyparallel to the direction in which the sheet was rolled.

This last treatment may be carried out either during the cooling stepfollowing the diffusion treatment, or separately, and the cooling rateduring this treatment in a magnetic field should range from 5 C. to 300C. per minute, according to the desired result and the nature of thetreated metal.

This treatment in a magnetic field combines very favorably with a hotplanishing treatment carried out after the static diffusion annealing.This hot planishing treatment is disclosed in applicants French Pat. No.1,426,093 of Nov. 22, 1963.

According to a specific arrangement of the method of this invention,optimum improvement in the magnetic properties of the sheet material isobtained, after the product subjected to the above process has beenshaped and just before its actual use, by applying at that time anotherheat treatment in a passage furnace at a temperature on the order of750-850 C. during a time period ranging from 1 to 3 minutes, without anyprotective atmosphere, this heat treatment being followed by coolingfrom 600 to 300 C. in a magnetic field of about 50 oersteds, thedirection of this magnetic field being parallel or substantiallyparallel to the preferential direction of easy magnetization of thesheet material to be treated.

By applying the method of this invention the watt loss characteristicsof the resulting product are improved from .10 to .20 watt per kilogramwith a 1.5 Tesla induction and a 50-cycle A.C. frequency.

From the foregoing it is clear that irrespective of the form ofembodiment contemplated, the mode of application and the meansimplemented therefor, there are obtained on the one hand a methodcapable of bringing substantial improvements in the magnetic propertiesof steels for magnetic applications, and on the other hand productsresulting from the use of such method which, in comparison with hithertoknown methods and magnetic products, aiming at the same purpose,provided substantial advantages, notably that of permitting theproduction of steel sheets for magnetic applications which have a totalcontent of at least one of aluminum, silicon, cobalt and germaniumhigher than what has previously been possible, so as to increase veryconsiderably the resistivity of the sheets obtained according to thisinvention and therefore reduce the watt losses of the sheets andincrease the efficiency of a treatment of directional order, withoutimpairing the other properties such as mechanical strength, fragility,etc. of the end product.

Although the present invention has been described in conjunction with apreferred embodiment, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyappreciate. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

What is claimed is:

1. A process for improving the magnetic properties of processed siliconsteel sheets oriented according to a structure belonging to the classincluding the edged cubical structure or the flat cubical structure,which comprises cleaning the faces of the sheet, coating the cleanedfaces of the sheet with a thin layer of at least one element selectedfrom the group consisting of aluminum, silicon, cobalt and germanium,diffusing the element into the sheet while heating the sheet at atemperature of about 750 to 900 C. for a time of up to hours in anatmosphere of dry hydrogen whereby said element is distributed in thesheet in a concentration decreasing from the faces to the core of thesheet, and treating the resultant sheet in a dry hydrogen atmosphere ina magnetic field of approximately 50 oersteds whose direction issubstantially parallel to the preferential direction of easymagnetization of the sheet, while cooling the sheet at decreasingtemperatures between 600 and 300 C.

2. The process according to claim 1, wherein subsequent to diffusing theelement into the sheet the sheet is submitted to magnetic treatment at acooling rate of 5 to 300 C. per minute.

3. The process according to claim 2, wherein the magnetic treatment iscarried out in combination with a hot planishing treatment.

4. The process according to claim 1, wherein the magnetic treatment iscarried out in combination with a hot planishing treatment.

5. The process according to claim 1, wherein the magnetic treatment isfollowed by a heat treatment at about 750 to 850 C. for a time between 1and 3 minutes in the absence of a protective atmosphere, said heattreatment then being followed by cooling from 600 to 300 C. in amagnetic field of approximately 50 oersteds whose direc tion issubstantially parallel to the direction of easy magnetization of thesheet.

6. The process according to claim 1, wherein the coating of each face ofthe sheet with the element is carried out employing a volatile compoundof the element.

7. The process according to claim 1, wherein the coating of each face ofthe sheet with the element is carried out by evaporating the element invacuo.

8. The process according to claim 1, wherein the element is aluminum andthe total quantity of the element coated on the sheet is 0.5 to 3% byweight of the sheet.

9. The process according to claim 1, wherein the element is cobalt andthe total quantity of the element coated on the sheet is 0.05 to 3% byweight of the sheet.

10. The process according to claim 1, wherein the sheet to be coated hasa thickness of about 0.35 mm. and the diffusion treatment is such thatthe product Dr, in C.G.S. units, is between 10- and 10* cm. where D isthe diffusion coefficient, in cm. /sec., of the element at the treatmenttemperature, and t is the time of the diffusion treatment in seconds.

(References on following page) 8 Retrences Cited 3,392,053 7/ 1968 Olsonet a] 117-212 2 307 605 1/1943 Ruder et a1. 175--356 T N S; Z ES f TS148 112 X FOREIGN PATENTS 1/19 F 18,068 9/1963 Japan 148-100 12/1968Glmlgllano 148112 X 5 4 97 3 19 4 Japan 17 2 4 5/1970 Ganz 148-112 X12/1965 Sixtus et a1 14s 112 HLLIAM MAR Pumary Examiner 10/1946Langworthy B. D. PIANALTO, Assistant Examiner 4/1962 1366166 et a1. 14s127 10 US. Cl. X.R. 11/1964 Dreher 148127 113

